diff --git a/TESTING.md b/TESTING.md
index 997bd28..2f5bc97 100644
--- a/TESTING.md
+++ b/TESTING.md
@@ -38,6 +38,6 @@ ctest --test-dir build-ubsan --output-on-failure
 ```
 
 The sanitizer flags shown are supported by Clang and GCC.  Other compilers may
-require different flags.  Debug compilation and a feasible solver run currently
-expose the pre-existing defects tracked by issues #7 and #14 respectively; the
-test additions deliberately do not include fixes for those separate issues.
+require different flags.  A feasible solver run currently exposes the
+pre-existing defect tracked by issue #14; the test additions deliberately do
+not include its separate fix.
diff --git a/main.cc b/main.cc
index ee2c9bd..c71cae1 100644
--- a/main.cc
+++ b/main.cc
@@ -85,7 +85,8 @@ namespace {
     auto set(std::string &s, size_t x, size_t y, char c) const noexcept -> void {
       assert(x < length_);
       assert(y < length_);
-      assert(grid_[x + y * length_] != c);
+      // Size labels may replace interior spaces, but must not duplicate a write.
+      assert(s[x + y * length_] != c);
       s[x + y * length_] = c;
     }